Amphiphilic Schiff base organogels: metal-ion-mediated chiral twists and chiral recognition

New amphiphilic gelators that contained both Schiff base and L-glutamide moieties, abbreviated as o-SLG and p-SLG, were synthesized and their self-assembly in various organic solvents in the absence and presence of metal ions was investigated. Gelation test revealed that o-SLG formed a thermotropic gel in many organic solvents, whilst p-SLG did not. When metal ions, such as Cu(2+), Zn(2+), Mg(2+), Ni(2+), were added, different behaviors were observed. The addition of Cu(2+) induced p-SLG to from an organogel. In the case of o-SLG, the addition of Cu(2+) and Mg(2+) ions maintained the gelating ability of the compound, whilst Zn(2+) and Ni(2+) ions destroyed the gel. In addition, the introduction of Cu(2+) ions caused the nanofiber gel to perform a chiral twist, whilst the Mg(2+) ions enhanced the fluorescence of the gel. More interestingly, the Mg(2+)-ion-mediated organogel showed differences in the fluorescence quenching by D- and L-tartaric acid, thus showing a chiral recognition ability.     

Magnesium fluctuations modulate RNA dynamics in the SAM-I riboswitch

Experiments demonstrate that Mg(2+) is crucial for structure and function of RNA systems, yet the detailed molecular mechanism of Mg(2+) action on RNA is not well understood. We investigate the interplay between RNA and Mg(2+) at atomic resolution through ten 2-μs explicit solvent molecular dynamics simulations of the SAM-I riboswitch with varying ion concentrations. The structure, including three stemloops, is very stable on this time scale. Simulations reveal that outer-sphere coordinated Mg(2+) ions fluctuate on the same time scale as the RNA, and that their dynamics couple. Locally, Mg(2+) association affects RNA conformation through tertiary bridging interactions; globally, increasing Mg(2+) concentration slows RNA fluctuations. Outer-sphere Mg(2+) ions responsible for these effects account for 80% of Mg(2+) in our simulations. These ions are transiently bound to the RNA, maintaining interactions, but shuttled from site to site. Outer-sphere Mg(2+) are separated from the RNA by a single hydration shell, occupying a thin layer 3-5 ? from the RNA. Distribution functions reveal that outer-sphere Mg(2+) are positioned by electronegative atoms, hydration layers, and a preference for the major groove. Diffusion analysis suggests transient outer-sphere Mg(2+) dynamics are glassy. Since outer-sphere Mg(2+) ions account for most of the Mg(2+) in our simulations, these ions may change the paradigm of Mg(2+)-RNA interactions. Rather than a few inner-sphere ions anchoring the RNA structure surrounded by a continuum of diffuse ions, we observe a layer of outer-sphere coordinated Mg(2+) that is transiently bound but strongly coupled to the RNA.     

Influence of Mg2+ and Cd2+ on the interaction between sparfloxacin and calf thymus DNA

Mg(2+) and Cd(2+) have different binding capacity to sparfloxacin, and have different combination modes with calf thymus DNA. Selecting these two different metal ions, the influence of them on the binding constants between SPFX and calf thymus DNA, as well as the related mechanism have been studied by using absorption and fluorescence spectroscopy. The result shows that Cd(2+) has weak binding capacity to SPFX in the SPFX-Cd(2+) binary system, but can decrease the binding between SPFX and DNA obviously in SPFX-DNA-Cd(2+) ternary system. Mg(2+) has strong binding capacity to SPFX. It can increase the binding between SPFX and DNA at concentrations <0.01 mM, and decrease the binding between them at concentrations >0.01 mM. Referring to the different modes of Mg(2+) and Cd(2+) binding to DNA, the mechanism of the influence of metal ions on the binding between SPFX and DNA has been proposed. SPFX can directly bind to DNA by chelating DNA base sites. If a metal ion at certain concentration mainly binds to DNA bases, it can decrease the binding constants between SPFX and DNA through competing with SPFX. While if a metal ion at certain concentration mainly binds to phosphate groups of DNA, it can increase the binding constants by building a bridge between SPFX and DNA. The influence direction of metal ions on the binding between quinolone and DNA relays on their binding ratio of affinity for bases to phosphate groups on DNA. Our result supports Palumbo's conclusion that the binding between SPFX and the phosphate groups is the precondition for the combination between SPFX and DNA, which is stabilized through stacking interactions between the condensed rings of SPFX and DNA bases.     

Synthesis and photoluminescence properties of Ce3+ and Eu2+-activated Ca7Mg(SiO4)4 phosphors for solid state lighting

Ce(3+) and Eu(2+) singly doped and Ce(3+)/Eu(2+)-codoped Ca(7)Mg(SiO(4))(4) phosphors are synthesized by the conventional solid state reaction. The Ce(3+) activated sample exhibits intense blue emission under 350 nm excitation, the composition-optimized Ca(7)Mg(SiO(4))(4)?:?4%Ce(3+) shows better color purity than the commercial blue phosphor, BaMgAl(10)O(17)?:?Eu(2+) (BAM?:?Eu(2+)) and exhibits superior external quantum efficiency (65%). The Ca(7)Mg(SiO(4))(4)?:?Eu(2+) powder shows a broad emission band in the wavelength range of 400-600 nm with a maximum at about 500 nm. The strong excitation bands of the Ca(7)Mg(SiO(4))(4)?:?Eu(2+) in the wavelength range of 250-450 nm are favorable properties for applications as light-emitting-diode conversion phosphors. Furthermore, the energy transfer from the Ce(3+) to Eu(2+) ions is observed in the codoped samples, the resonance-type energy transfer is determined to be due to the dipole-dipole interaction mechanism and the critical distance is obtained through the spectral overlap approach and concentration quenching method.     

Effect of the ionic strength and prostaglandin E2 on the free Ca2+ concentration and the Ca2+ influx in human red blood cells

Human red blood cells (RBCs) were loaded with the Ca(2+)-sensitive fluorescent dye fura-2 to investigate the effects of media ionic strength and prostaglandin E2 (PGE2) on the intracellular free Ca2+ concentration ([Ca2+]i). [Ca2+]i of intact RBCs in a Ca(2+)-containing physiological (high) ionic strength (HIS) solution was 75.1 +/- 8.3 nM after 5 min incubation, increasing to 114.9 +/- 9.6 nM after 1 h. In Ca(2+)-containing low ionic strength (LIS) solutions, [Ca2+]i was significantly lower than in the Ca(2+)-containing HIS solution (p = 0.041 or 0.0385 for LIS solutions containing 200 or 250 mM sucrose, respectively), but, as in HIS solution, an increase of [Ca2+]i was seen after 1 h. In Ca(2+)-free (0 Ca2+ plus 15 microM EGTA) media, [Ca2+]i decreased (ranging from 15 to 21 nM), but were not significantly different in HIS or LIS, and did not change following 1 h incubation. The effect of the ionic strength and PGE2 on passive Ca2+ influx was investigated on ATP-depleted RBCs. Ca2+ influx was faster during the initial 10 min in comparison with the subsequent time period (10-45 min), both in HIS and LIS media, decreasing from 20.3 +/- 1.9 to 12.9 +/- 1.3 micromol/(lcells x h) in HIS, and from 36.7 +/- 5.3 to 8.6 +/- 1.2 micromol/(lcells x h) in LIS. Prostaglandin E2 (PGE2; 10(-7)-10(-11) M), dissolved in deionised water or in ethanol, did not affect [Ca2+]i in either normal or in ATP-depleted RBCs suspended in Ca(2+)-containing HIS medium. Finally, the addition of carbachol (100 microM) did not affect [Ca2+]i. The present findings suggest that stimulation of the Ca(2+)-activated K+ channel by PGE2, reported in [J. Biol. Chem. 271 (1996) 18651], cannot be mediated via increased [Ca2+]i.     

Competition between Li+ and Mg2+ in metalloproteins. Implications for lithium therapy

Lithium is used (in the form of soluble salts) to treat bipolar disorder and has been considered as a possible drug in treating chronic neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's diseases. One of the proposed mechanisms of Li(+) action involves a competition between the alien Li(+) and native Mg(2+) for metal-binding sites and subsequent inhibition of key enzymes involved in specific neurotransmission pathways, but not vital Mg(2+) proteins in the cell. This raises the following intriguing questions: Why does Li(+) replace Mg(2+) only in enzymes involved in bipolar disorder, but not in Mg(2+) proteins essential to cells? In general, what factors allow monovalent Li(+) to displace divalent Mg(2+) in proteins? Specifically, how do the composition, overall charge, and solvent exposure of the metal-binding site as well as a metal-bound phosphate affect the selectivity of Li(+) over Mg(2+)? Among the many possible factors, we show that the competition between Mg(2+) and Li(+) depends on the net charge of the metal complex, which is determined by the numbers of metal cations and negatively charged ligands, as well as the relative solvent exposure of the metal cavity. The protein itself is found to select Mg(2+) over the monovalent Li(+) by providing a solvent-inaccessible Mg(2+)-binding site lined by negatively charged Asp/Glu, whereas the cell machinery was found to select Mg(2+) among other competing divalent cations in the cellular fluids such as Ca(2+) and Zn(2+) by maintaining a high concentration ratio of Mg(2+) to its biogenic competitor in various biological compartments. The calculations reveal why Li(+) replaces Mg(2+) only in enzymes that are known targets of Li(+) therapy, but not in Mg(2+) enzymes essential to cells, and also reveal features common to the former that differ from those in the latter proteins.     

The effect of magnesium ions on vitamin D(2)-phospholipid model membrane interactions in the presence of different buffer media

Vitamin D plays important roles in the bone formation, in calcium and phosphorus homeostasis and in the treatment and prevention of many diseases. Ions, especially divalent cations like Mg(2+), have indispensable roles in many vital biological events. Mg(2+) is involved in many fundamental processes such as stabilization of membranes and macromolecules, synthesis of nucleic acid and proteins and formation and use of high-energy phosphate bonds. Mg(2+) is also required for synthesis of more than 310 different enzymes of the body and is, therefore, involved in many important activities. The roles of vitamin D and major ions in the body are quite well known. While there are still many unresolved points about the exact molecular mechanism behind such diverse functions, in the present study, the interaction of Mg(2+) with dipalmitoyl phosphatidylcholine (DPPC) model membranes has been studied in the presence and absence of vitamin D(2) by using Fourier transform infrared (FTIR) spectroscopy and turbidity technique at 440 nm. The effect of different buffer media on the system has also been investigated. The temperature dependent investigation of the wavelength of the CH(2) antisymmetric stretching bands revealed that, in the presence of N-[2-hydroxyethyl] piperazine-N'-[2-ethanesulfonic acid] (Hepes) and phosphate buffer, addition of Mg(2+) and/or vitamin D(2) into pure DPPC liposomes does not change the shape of the phase transition profile. Turbidity studies support these results. In the presence of Hepes buffer, the inclusion of Mg(2+) and/or vitamin D(2) into pure DPPC liposomes orders the system. In the presence of phosphate buffer, FTIR study showed that, addition of Mg(2+) into pure DPPC liposomes disorders the system in the gel phase. The precipitation of Mg(2+) with phosphates, which is present in phosphate buffer, may be a reason for this difference in the effect. It is seen that, the binary mixture of Mg(2+)-DPPC and the ternary mixture of Mg(2+)-vitamin D(2)-DPPC behave differently in the presence of two different buffer media.     

Cation-dependent switching of DNA nanostructures

DNA is a versatile building material for nanoconstruction because of its remarkable molecular-recognition capability and well-predicted duplex conformation. A number of DNA motifs have been engineered, which can assemble into well-defined nanostructures in Mg(2+)-containing buffer solution. XRD studies reveal that the DNA conformation is slightly influenced by divalent cations (such as Mg(2+) or Ca(2+)). This phenomenon can be utilized in DNA self-assembly for regulating self-assembled DNA nanostructures. As an initial step, a symmetric cross motif forms flat, periodic, 2D lattices in Mg(2+)-containing solutions, but long nanofibers in Ca(2+)-containing solutions. The obtained DNA fibers can serve as templates to fabricate CaCO(3) nanotubes and nanowires.     

Purification and Biochemical Characterization of Methionine Aminopeptidase (MetAP) from Mycobacterium smegmatis mc2155

The methionine aminopeptidase (MetAP) catalyzes the removal of amino terminal methionine from newly synthesized polypeptide. MetAP from Mycobacterium smegmatis mc(2) 155 was purified from the culture lysate in four sequential steps to obtain a final purification fold of 22. The purified enzyme exhibited a molecular weight of approximately 37 kDa on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Activity staining was performed to detect the methionine aminopeptidase activity on native polyacrylamide gel. The enzyme was characterized biochemically, using L-methionine p-nitroanilide as substrate. The enzyme was found to have a temperature and pH optimum of 50 degrees C and 8.5, respectively, and was found to be stable at 50 degrees C with half-life more than 8 h. The enzyme activity was enhanced by Mg(2+) and Co(2+) and was inhibited by Fe(2+) and Cu(2+). The enzyme activity inhibited by EDTA is restored in presence of Mg(2+) suggesting the possible role of Mg(2+) as metal cofactor of the enzyme in vitro.     

Analysis of the role of Mg²⁺ on conformational change and target recognition by ciliate Euplotes octocarinatus centrin

The binding of Mg(2+) with the Euplotes octocarinatus centrin (EoCen) and the effect of Mg(2+) on the binding of EoCen with the peptide melittin were examined by spectroscopic methods. In this study, it was found that Mg(2+) may bind with Ca(2+)-binding sites, at least partly, on EoCen, which displays ～10-fold weaker affinity than Ca(2+). In the presence of Mg(2+), Ca(2+)-saturated EoCen undergoes significant conformational changes resulting in decreased exposure of hydrophobic surfaces on the protein. Additionally, excess Mg(2+) did not change the stoichiometry, but rather reduced the affinity of EoCen to melittin. The Mg(2+)-dependent decrease in the affinities of EoCen to melittin is an intrinsic property of Mg(2+), rather than a nonspecific ionic effect. The inhibitory effect of Mg(2+) on the formation of complexes between EoCen and melittin may contribute to the specificity of EoCen in target activation in response to cellular Ca(2+) concentration fluctuations.     

Ultraviolet photodepletion spectroscopy of dibenzo-18-crown-6-ether complexes with alkaline Earth metal divalent cations

Ultraviolet photodepletion spectra of dibenzo-18-crown-6-ether complexes with alkaline earth metal divalent cations (A(2+)-DB18C6, A = Ba, Sr, Ca, and Mg) were obtained in the gas phase using electrospray ionization quadrupole ion-trap reflectron time-of-flight mass spectrometry. Each spectrum exhibits the lowest energy absorption band in the wavenumber region of 35?400-37?800 cm(-1), which is tentatively assigned as the origin of the S(0)-S(1) transition of A(2+)-DB18C6. This origin band shows a red shift as the size of the metal dication increases from Mg(2+) to Ba(2+). The binding energies of the metal dications to DB18C6 at the S(0) state were calculated at the lowest energy structures optimized by the density functional theory and employed with the experimental energies of the origin bands to estimate the binding energies at the S(1) state. We suggest that the red shifts of the origin bands arise from the decrease in the binding energies of the metal dications at the S(1) state by nearly constant ratios with respect to the binding energies at the S(0) state, which decrease with increasing size of the metal dication. This unique relationship of the binding energies between the S(0) and S(1) states gives rise to a linear correlation between the relative shift of the origin band of A(2+)-DB18C6 and the binding energy of the metal dication at the S(0) state. The size effects of the metal cations on the properties of metal-DB18C6 complex ions are also manifested in the linear plot of the relative shift of the origin band as a function of the size to charge ratio of the metal cations, where the shifts of the origin bands for all DB18C6 complexes with alkali and alkaline earth metal cations are fit to the same line.     

Ca2+ selectivity of the sarcoplasmic reticulum Ca2+-ATPase at the enzyme-water interface and in the Ca2+ entrance channel

The sarcoplasmic reticulum (SR) Ca(2+)-ATPase, a P-type transmembrane protein, can transport Ca(2+) from the cytoplasmic to the luminal side over other cations specifically. The proposed Ca(2+) entrance channel, composed of the main-chain carbonyl oxygen and side-chain carboxyl oxygen atoms of the amino acids, opens on the enzyme surface, just above the biphospholipid layer membrane-water interface, where Trp residues are frequently found. In this work, the physicochemical nature of Ca(2+) selectivity over Mg(2+) on the surface of the SR Ca(2+)-ATPase has been investigated using the density functional theory (DFT) method. The selection process can be regarded as the first step of the specificity of the enzyme to transport Ca(2+). Subsequently, the specificity of the entrance channel to conduct Ca(2+) over other cations has also been explored. As revealed by thermodynamic analyses, either the aromatic or the aliphatic amino acid residues distributed on the surface of Ca(2+)-ATPase have a bigger affinity to Mg(2+) than to Ca(2+), resulting in a concentration decrease of free Mg(2+) in the local region. Thus, Ca(2+) can transport into the Ca(2+)-entrance channel more easily. Whereafter, for a small quantity of Mg(2+) entering this channel accompanying the Ca(2+) current, the strong electrostatic interactions between Mg(2+) and the ligands will limit the activity of this metal ion, which facilitates the weakly bonded Ca(2+) passing through the channel at a relatively high rate, as suggested by the "sticky-pore" hypothesis. Furthermore, the corresponding theoretical investigations have demonstrated that the increase of the ligand electronegativity can enhance their discrimination between these two cations effectively.     

Polymerized crystalline colloidal array chemical-sensing materials for detection of lead in body fluids

We have developed intelligent polymerized crystalline colloidal array (IPCCA) chemical-sensing materials for detection of Pb(2+) in high ionic-strength environments such as body fluids with a detection limit of <500 nmol L(-1) Pb(2+) (100 ppb). This IPCCA lead sensor consists of a mesoscopically periodic array of colloidal particles polymerized into an acrylamide hydrogel. The array Bragg-diffracts light in the visible spectral region because of the periodic spacing of the colloidal particles. This material also contains a crown ether chelating agent for Pb(2+). Chelation of Pb(2+) by the IPCCA in low-ionic-strength solutions results in a Donnan potential that swells the gel, which red-shifts the diffracted light in proportion to the Pb(2+) concentration. At high ionic strength the Donnan potential is, unfortunately, swamped and no static response occurs for these sensors. We demonstrate, however, that we can determine Pb(2+) at high ionic strength by incubating these IPCCA in a sample solution and then measuring their transient response on exposure to pure water. The non-complexed ions diffuse from the IPCCA faster than the bound Pb(2+). The resulting transient IPCCA diffraction red-shift is proportional to the concentration of Pb(2+) in the sample. These IPCCA sensors can thus be used as sensing materials in optrodes to determine Pb(2+) in high-ionic-strength solutions such as body fluids.     

Control of photoinduced electron transfer in zinc phthalocyanine-perylenediimide dyad and triad by the magnesium ion

Photoexcitation of a zinc phthalocyanine-perylenediimide (ZnPc-PDI) dyad and a bis(zinc phthalocyanine)-perylenediimide [(ZnPc) 2-PDI] triad results in formation of the triplet excited state of the PDI moiety without the fluorescence emission, whereas addition of Mg (2+) ions to the dyad and triad results in formation of long-lived charge-separated (CS) states (ZnPc (*+)-PDI (*-)/Mg (2+) and (ZnPc) 2 (*+)-PDI (*-)/Mg (2+)) in which PDI (*-) forms a complex with Mg (2+). Formation of the CS states in the presence of Mg (2+) was confirmed by appearance of the absorption bands due to ZnPc (*+) and PDI (*-)/Mg (2+) complex in the time-resolved transient absorption spectra of the dyad and triad. The one-electron reduction potential ( E red) of the PDI moiety in the presence of a metal ion is shifted to a positive direction due to the binding of Mg (2+) to PDI (*-), whereas the one-electron oxidation potential of the ZnPc moiety remains the same. The binding of Mg (2+) to PDI (*-) was confirmed by the ESR spectrum, which is different from that of PDI (*-) without Mg (2+). The energy of the CS state (ZnPc (*+)-PDI (*-)/Mg (2+)) is determined to be 0.79 eV, which becomes lower that of the triplet excited state (ZnPc- (3)PDI*: 1.07 eV). This is the reason why the long-lived CS states were attained in the presence of Mg (2+) instead of the triplet excited state of the PDI moiety.     

Thin-layer chromatographic behavior and separation of alkaline earth metals on silica gel in aqueous sodium perchlorate solution

The thin-layer chromatographic behavior of alkaline earth metal ions (Mg2+, Ca2+, Sr2+, and Ba2+) in aqueous sodium perchlorate solutions on silica gel thin-layers impregnated with sodium hydroxide has been surveyed as a function of salt concentration. At salt concentrations above 2 mol 1(-1), the selectivity of the metals increased with a decrease in the crystal ionic radii; with further increases in salt concentration, the selectivity differences among the metals expanded remarkably. In the present systems, it was supposed that the cation exchange, the surface complexation, and the salting-out effect participate simultaneously in the adsorption of the metals on silica gel. Typical chromatograms for the mutual separation of the alkaline earth metals are presented.     

Characterisation of Ni silicate-bearing minerals by UV-vis-NIR spectroscopy. Effect of Ni substitution in hydrous Ni-Mg silicates

Three Ni silicate-bearing pimelite, nepouite and pecoraite minerals, from Australia have been investigated by UV-vis-NIR spectroscopy to study the effect of Ni-Mg substitution. The observation of three major absorption bands at 9205-9095, 15,600-15,190 and 26,550-25,660 cm(-1) are the characteristic features of Ni(2+) in sixfold coordination. The effect of cation substitution like Mg(2+) for Ni(2+) on band shifts in electronic and vibrational spectra enable the distinction between the Ni-bearing silicates.     

A metallic cobalt electrode for the indirect potentiometric determination of calcium and magnesium in natural waters using flow injection analysis

A flow injection analysis of Ca(2+) and Mg(2+) using indirect potentiometric detection in natural waters is proposed, where Ca(2+) or Mg(2+) are injected into a buffer carrier containing phosphate, resulting in the formation of Ca(3)(PO(4))(2) or Mg(3)(PO(4))(2). The consequent reduction in free phosphate in the carrier solution is detected using a metallic cobalt wire electrode. Indirect electrode response was used and the experimental conditions affecting electrode response were optimized. Responses were linear in the concentration range 5x10(-4) to 5x10(-3) M with a detection limit of 1x10(-5) M in 20 mM phosphate buffer at pH 8.0. The relative standard derivation at 1 mM of Ca(2+) and Mg(2+) were 3.9 and 3.7% (n=10), respectively. EGTA and 8-hydroxyquinoline were used as the masking agents for Ca(2+) and Mg(2+), respectively. Concentrations of Ca(2+) and Mg(2+) in natural waters were successfully determined by the proposed method.     

Novel 15-crown-5 ether or beta-diketone incorporated gadolinium complexes for the detection of potassium ions or magnesium and calcium ions

Novel gadolinium complexes (KMR-series: KMR-K and KMR-Mg), which have a bis-15-crown-5 ether or a charged beta-diketone structure as a recognition site, have been designed, synthesized and applied for the detection of K(+) or of Mg(2+) and Ca(2+) using MRI or NMR techniques. The measurements are based on the modulation of the longitudinal relaxation time (T(1)) of water protons in proximity of the gadolinium complexes. Relaxivity measurements of KMR-K1 in aqueous solution showed that the initial longitudinal relaxivity value (r(1)) of 5.05 mM(-1) s(-1) is monotonously decreasing with increasing K(+) concentrations, reaching a final value of 4.78 mM(-1) s(-1). This decrease is attributed to a change in the second sphere of hydration of the gadolinium (Gd(3+)) complex (KMR-K), resulting in a K(+) concentration-dependent contrast in MR images. From stoichiometric analysis using mass spectrometry and UV/VIS spectrometry, a 1 : 1 complex formation between KMR-K1 and K(+) in a sandwich-type manner with a log K of 3.20 was confirmed. In the case of KMR-Mg, the initial r(1) value of 4.98 mM(-1) s(-1) is monotonously decreasing with increasing Mg(2+) or Ca(2+) concentrations, reaching a final value of 3.95 or 4.16 mM(-1) s(-1), respectively, resulting in Mg(2+) and Ca(2+) concentration-dependent contrast in MR images. The formation of a 1:1 complex with a log K of 2.33 for Mg(2+) and 1.91 for Ca(2+) was confirmed. KMR-K1 and KMR-Mg are the first ion-selective or ion-sensitive gadolinium complexes for K(+) or Mg(2+) and Ca(2+), respectively.     

Phase behavior of aqueous suspensions of Mg(2)Al layered double hydroxide: the competition among nematic ordering, sedimentation, and gelation

Birefringence observations and rheological measurements were used to monitor the phase behavior of Mg/Al (the molar ratio of Mg(2+) to Al(3+) being 2:1) layered double hydroxide (LDH) suspensions. The suspensions of concentration lower than 16% (w/w) appear isotropic (I) between crossed polarizers. In contrast, the suspensions of concentration between 16% and 30% (w/w) showed an isotropic (I)-nematic (N) biphasic coexistence. Detailed observations led us to divide the suspensions in the gap into three groups according to their behaviors: the suspensions with concentration between 16% and 25% (w/w) experienced an I-N phase transition and particle sedimentation simultaneously, while the suspensions of 25% to 27% (w/w) showed I-N transition after particle sedimentation, and in the suspension of 30% (w/w), a critical sol-gel transition appeared with an I-N transition. Above 33% (w/w), the gel network hindered a complete I-N separation in the suspensions. Upon raising the NaCl concentration, the liquid crystalline phase transition and the sol-gel transition shifted to higher particle concentrations. The facts demonstrate that the phase behavior of aqueous LDH suspensions is controlled by the competition among liquid crystal phase transition, sedimentation, and gelation.     

Moving blue band caused by cooperation of diffusion and phase separation

Diffusions of Cu(2+) and Fe(3+) in gelatin generate a moving blue band. It is formed by a diffusion of Cu(2+) and a phase separation of gelatin with diffusing Fe(3+). The diffusing Fe(3+) forms Fe(OH)(3) colloids and gathers gelatin molecules from the surroundings. The diffusion of gelatin molecules generates the concentration gradient, resulting in a gel/sol transition in the dilute phase. In the region where the concentration of Fe(3+) is high enough, the gel remains hard, while a sol phase appears under the hard gel. The absorption spectrum of Cu(2+) depends on the concentration ratio of Cu(2+) to gelatin. As a consequence, we can see a blue band in the restricted region between the diffusing front of Cu(2+) and the phase separation front. The movement of the blue band is caused by a coupling of a simple diffusion and the phase separation.